Oxide superconductors of the rare earth-barium--copper-oxide family (YBCO), bismuth(lead)--strontium--calcium--copper-oxide family ((Bi,Pb)SCCO) and thallium--barium--calcium--copper-oxide family (TBCCO) form plate-like and highly anisotropic superconducting oxide grains. Because of their plate-like morphology, the oxide grains can be oriented by mechanical strain. Mechanical deformation has been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. The degree of alignment of the oxide superconductor grains is a major factor in the high critical current densities (J.sub.c) obtained in articles prepared from these materials.
Known processing methods for obtaining textured oxide superconductor composite articles include an iterative process of alternating heating and deformation steps. The heat treatment is used to promote reaction-induced texture of the oxide superconductor in which the anisotropic growth of the superconducting grains is enhanced. Each deformation provides an incremental improvement in the orientation of the oxide grains. Additional heat treatment intermediate with or subsequent to deformation is also required to form the correct oxide superconductor phase, promote good grain interconnectivity and achieve proper oxygenation.
Processing long lengths of oxide superconductor is particularly difficult because deformation leads to microcracking and other defects which may not be healed in the subsequent heat treatment. Cracks that occur perpendicular to the direction of current flow limit the performance of the superconductor. In order to optimize the current carrying capability of the oxide superconductor, it is necessary to heal any microcracks that are formed during processing of the oxide superconductor or superconducting composite.
Liquid phases in co-existence with solid oxide phases have been used in processing of oxide superconductors. One type of partial melting, known as peritectic decomposition, takes advantage of liquid phases which form at peritectic points of the phase diagram containing the oxide superconductor. During peritectic decomposition, the oxide superconductor remains a solid until the peritectic temperature is reached, at which point the oxide superconductor decomposes into a liquid phase and a new solid phase. The peritectic decompositions of Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x, (BSCCO-2212, where 0.ltoreq.x.ltoreq.1.5), into an alkaline earth oxide and a liquid phase and of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO-123, where 0.ltoreq..delta..ltoreq.1.0) into Y.sub.2 BaCuO.sub.5 and a liquid phase are well known.
Peritectic decomposition of an oxide superconductor and the reformation of the oxide superconductor from the liquid+solid phase is the basis for melt textured growth of YBCO-123 and BSCCO-2212. For example, Kase et al. in IEEE Trans. Mag. 27(2), 1254 March 1991) report obtaining highly textured BSCCO-2212 by slowly cooling through the peritectic point. This process necessarily involves total decomposition of the desired 2212 phase into an alkaline earth oxide and a liquid phase.
It is also recognized that an oxide superconductor itself can co-exist with a liquid phase under suitable processing conditions. This may occur by solid solution melting, eutectic melting or by formation of non-equilibrium liquids.
Solid solution melting may arise in a single phase system, in which the oxide superconductor is a solid solution. As the temperature (or some other controlling parameter) of the system increases (or decreases), the oxide superconductor phase changes from a solid oxide phase to a liquid plus oxide superconductor partial melt (this happens at the solidus). A further increase in temperature (or some other controlling parameter) affords the complete melting of the oxide superconductor (this happens at the liquidus).
A phase diagram containing a eutectic point may provide an oxide superconductor partial melt, known as eutectic melting, when the overall composition is chosen so as to be slightly off stoichiometry. As the temperature (or some other controlling parameter) of the system increases (or decreases), the mixed phase of oxide superconductor-plus-non-superconducting oxide (solid.sub.1 +solid.sub.2) changes to a liquid-plus-oxide superconductor partial melt (solid.sub.1 +liquid).
Non-equilibrium liquids may also promote partial melting in oxide superconductor systems. A non-equilibrium liquid is established through the relatively rapid heating of a mixture of oxides to a temperature above the eutectic melting point of local stoichiometries present in the heterogeneous mixture of phases. As the oxides form the desired oxide superconductor, the solid and liquid phases can co-exist, if only temporarily.
Partial melting of (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+x ((Bi,Pb)SCCO-2223, where 0.ltoreq.x.ltoreq.1.5) and (Bi).sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.10+x ((Bi)SCCO-2223, where 0.ltoreq.x.ltoreq.1.5), collectively BSCCO-2223, at temperatures above 870.degree. C. in air has been reported; see, for example, Kobayashi et al. Jap. J. Appl. Phys. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055 (November 1988), Luo et al. Appl. Super. 1, 101-107, (1993), Aota et al. Jap. J. Appl. Phys. 28, L2196-L2199 (1989) and Luo et al. J. Appl. Phys. 72, 2385-2389 (1992). The exact mechanism of partial melting of BSCCO-2223 has not been definitively established.
Guo et al. in Appl. Supercond. 1(1/2), 25 (January 1993) have described a phase formation-decomposition-reformation (PFDR) process, in which a pressed sample of (Bi,Pb)SCCO-2223 is heated above its liquidus to decompose the 2223 phase, followed by a heat treatment at a temperature below the solidus. The sample was subsequently pressed again and reannealed. The final anneal of the PFDR process includes a standard single step heat treatment in which there is no melting.
The "high T.sub.c " oxide superconductor Bi.sub.2-y Pb.sub.y Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+x, where 0.ltoreq.x.ltoreq.1.5 and 0.ltoreq.y.ltoreq.0.6 (BSCCO-2223 and (Bi,Pb)SCCO-2223, hereinafter referred to as "BSCCO-2223" to indicate both lead-doped and undoped compositions), is particularly desirable because of its high critical transition temperature (T.sub.c .about.110 K) and high critical current (I.sub.c, J.sub.c). The superconducting art constantly seeks to improve electrical properties, such as, critical current density and critical transition temperature.
Partial melting in the processing of oxide superconductors has been used either to increase the yield of the BSCCO-2223 phase or to improve the contiguity and texturing of the oxide superconductor grains. The issue of healing defects, such as microcracks, which develop during processing of the oxide superconductor, has not been addressed. Further, the prior art has not addressed the possibility of using a two-step process where the oxide superconductor is stable in both steps for the healing of cracks and defects.
Wang et al. ("Advances in Superconductivity", Springer-Verlag, New York, Editors: Y. Bando and H. Yamauchi, pp. 291-294 (1993)) report an increase in T.sub.c by carrying out a post-anneal step at 790.degree. C. at reduced total pressures. Wang et al. observed T.sub.c by DC magnetization of 115 K and T.sub.c, zero of 111 K by resistivity measurement. The technique used by Wang et al. (vacuum encapsulation at 10.sup.-4 Torr of oxide superconductor pellets, followed by annealing at 790.degree. C.) does not permit determination of the oxygen pressure of the system. The encapsulated pellets reach an equilibrium oxygen pressure within the capsule by releasing oxygen. The pellet volume/capsule volume plays an important role in determining the final equilibrium oxygen pressure.
Critical transition temperatures (determined by magnetization) as high as 117 K have been reported for multiphase materials containing BSCCO-2223. Fisher et al. (Physica C 160, 466 (1990)) reported a T.sub.c of 115 K (determined by magnetization) with the substitution of lead and antimony in the BSCCO-2223 system. A non-reproducible T.sub.c as high as 130 K was reported by Hongbo et al. (Solid State Comm. 69, 867 (1989)).
While reports of high transition temperatures by magnetization studies are of interest, they can sometimes be misleading. The transition curves obtained by magnetization are "soft", making extrapolation to zero resistivity highly subjective. Further, other effects, such as semiconductor to metallic transitions, can mimic critical temperature transition behavior. It is therefore desirable to rely on bulk resistivity measurements for determining temperature at zero resistivity (T.sub.c, zero)
Idemoto et al. (Physica C 181, 171-178 (1991)) has investigated the oxygen content and copper and bismuth valances of BSCCO-2223 under a range of conditions, including temperatures in the range of 500.degree. C. to 850.degree. C. and oxygen pressures in the range of 0.005 to 0.20 atm. The samples were observed by means of a microbalance under changing temperatures conditions at constant oxygen pressures. Because the samples do not reach equilibrium during the observation period, it is difficult to determine the exact processing conditions experienced by the samples. No investigation of the effect of reported conditions on electrical properties is reported.
None of the previous research has indicated the desirability of post-annealing the BSCCO-2223 phase at low temperatures and oxygen pressures to enhance the electric transport properties of the oxide superconductor, namely critical current.
It is the object of the present invention to provide a method for improving superconducting performance of oxide superconductors and superconducting composites by healing cracks and defects formed during processing of oxide superconductors and superconducting composites.
It is a further object of the invention to prepare oxide superconducting articles having significantly less cracks and defects than conventionally-processed articles.
A further object of the present invention is to provide a process to increase the critical current density of BSCCO-2223 by a method which also increases its critical transition temperature. It is a further object of the present invention to provide a novel high-T.sub.c BSCCO-2223 composition having a critical transition temperature greater than 111.0 K.
A feature of the invention is a two-step heat treatment after which no further deformation occurs which introduces a small amount of a liquid phase co-existing with the oxide superconductor phase, and then transforms the liquid back into the oxide superconductor phase with no deformation occurring during or after the heat treatment of the invention. A further feature of the present invention is a low temperature, low oxygen pressure anneal of the oxide superconductor.
An advantage of the invention is the production of highly defect-free oxide superconductor and superconducting composites which exhibit superior critical current densities. A further advantage of the invention is a marked improvement in critical transition temperature and critical current density as compared to oxide superconductors and superconducting composites which are not subjected to the method of the invention.